Consumers use paper products, such as facial tissues, bath tissues, and paper towels, for a wide variety of applications. Facial tissues are not only used for nose care but, in addition to other uses, can also be used as a general wiping product. Consequently, there are many different types of tissue products currently commercially available.
In some applications, paper products are treated with lotions and/or various other additives for numerous desired benefits. For example, formulations containing polysiloxanes have been topically applied to tissue products in order to increase the softness of the product. In particular, adding silicone compositions to a facial tissue can impart improved softness to the tissue while maintaining the tissue's strength. For example, polysiloxane treated tissues are described in U.S. Pat. Nos. 4,950,545; 5,227,242; 5,558,873; 6,054,020; 6,231,719 and 6,432,270 and which are incorporated by reference herein. A variety of substituted and non-substituted polysiloxanes can be used.
While polysiloxanes are exceptionally good at improving softness there are drawbacks to their use. Polysiloxanes are generally hydrophobic, that is, they tend to repel water. Tissue products treated with polysiloxane tend to be less absorbent than tissue products not containing polysiloxane. Hydrophilic polysiloxanes are known in the art, however, such hydrophilic polysiloxanes are more water soluble and hence when applied to a tissue sheet will tend to migrate more in the z-direction of the sheet than the hydrophobic polysiloxanes. This means that less polysiloxane is available on the surface of the tissue product at a given addition level. Hence, higher levels of hydrophilic polysiloxanes are required to achieve the same level of softness as hydrophobic polysiloxanes. Hydrophilic polysiloxanes are also usually sold at a cost premium to the hydrophobic polysiloxanes. Therefore, hydrophilic polysiloxanes tend to be less effective at softening and more costly to use than hydrophobic polysiloxanes.
Polysiloxanes effective in providing surface softness to the sheet also tend to be poorly retained in the wet end of the tissue making process. Hence, to get the most benefit topical application to a formed tissue sheet is usually required. This topical application requires significant capital expense or machine modifications to employ in existing processes not set to employ topical application of polysiloxanes.
In co-pending U.S. application Ser. No. 09/802,529 filed Apr. 3, 2001 by Runge, et. al., a method for preparing fibers containing hydrophobic entities, including hydrophobic polysiloxanes, at a pulp mill is disclosed. These so called “polysiloxane pretreated fibers” can then be re-dispersed in the wet end of a paper-making process to manufacture paper products containing polysiloxane. It has been found that fibers treated with polysiloxane and dried prior to being re-dispersed and formed into a tissue sheet demonstrate excellent retention of the polysiloxane through the tissue making process. Unfortunately, use of these pretreated fibers in tissue products can lead to unacceptably high levels of hydrophobicity even when low levels of polysiloxane are used. In certain cases, the degree of hydrophobicity introduced into the sheet using polysiloxane pretreated fibers is greater than when the same level of polysiloxane is topically applied to the sheet by the methods known in the art.
Increased hydrophobicity in a paper product, such as a tissue, can adversely impact upon the ability of the wiping product to absorb liquids. Hydrophobic agents can also prevent bath tissue from being wetted in a sufficient amount of time and prevent disintegration and dispersing when disposed in a commode or toilet.
On the other hand, increasing the hydrophobicity of a paper web does provide various advantages. For example, by making the web hydrophobic, the fluid strike-through properties of the tissue product are improved. In other words, fluids absorbed by the web remain on the interior of the web and thus do not transfer to the hands of a user. Hydrophobic tissue products prepared using standard cellulose sizing agents are described in U.S. Pat. No. 6,027,611 issued to McFarland, et.al., and incorporated by reference herein. However, those skilled in the art will recognize the difficulties associated with using sizing agents to control hydrophobicity to a level acceptable for tissue products, the addition often resulting in products having unacceptably high levels of hydrophobicity. Furthermore, addition of sizing agents as described by McFarland, et.al., does not allow for regions of high and low hydrophobicity in the sheet but rather creates a uniformly hydrophobic sheet. Hence, additives that are hydrophobic in nature can make it difficult to find a proper balance between improving the properties of a web through the use of the additive and yet maintaining acceptable absorbency and wetability characteristics.
It is known to add a wetting agent directly to a polysiloxane emulsion then topically apply the polysiloxane, wetting agent composition to the tissue sheet to mitigate the hydrophobicity caused by addition of the polysiloxane. While this perhaps reduces the overall hydrophobicity of the sheet it does not allow for making tissues having uniform polysiloxane coverage with alternating hydrophobic and hydrophilic regions. Furthermore, combination of wetting agents with polysiloxane precludes application of the polysiloxane prior to the tissue making process. As the wetting agents are water soluble or water dispersible they are prone to loss during the tissue making process and, hence, the finished tissue sheet maintains its hydrophobicity.
It is also known to topically apply hydrophobic additives in discrete locations on a tissue sheet in conjunction with relatively large untreated areas of the sheet such that less than 50% of the surface of the sheet is covered with the additive. Such discrete placement of the additive on the tissue sheet is expected to provide regions of hydrophobicity and hydrophilicity. However, such discrete placement requires a majority of the tissue surface to not contain the additive. As a result, reduced product benefits, such as softness, are realized relative to a sheet having a high level of surface coverage. Furthermore, this process precludes use of hydrophobic additives prior to the tissue sheet forming step. Hence, processes that employ applying the hydrophobic additive in discrete locations on the tissue sheet surface, preclude addition of the hydrophobic additive to the fiber slurry in the wet-end of the tissue process or addition of the hydrophobic additive as pretreated fibers. Addition of the hydrophobic additive prior to the tissue forming process, either in the wet end fiber slurry or as pretreated fibers, is preferred since minimum added capital cost is needed for employment on existing tissue assets.
U.S. Pat. Nos. 6,238,519 and 6,458,243 issued to Jones, et.al, describe the use of deactivated ketene dimer agents to reduce the hydrophobicity of sheets relative to those made with standard alkyl ketene dimers. While lower hydrophobicity is noted, the application precludes formation of specific regions of hydrophobicity and hydrophilicity, hence, the application of deactivated ketene dimers does not allow for fine tuning control of hydrophobic and hydrophilic properties.
Thus, a need currently exists for tissue products and methods to prepare tissue products containing hydrophobic additives wherein the hydrophobic additive is present across a majority of the sheet surface, yet the benefits to the product are provided without increasing the hydrophobicity of the product beyond desirable limits. Furthermore there is a need to produce such products in a manner that the hydrophobic additive may be applied prior to the sheet forming step in the wet end of the tissue process or as pretreated pulp fibers. There is furthermore a need for tissue products and processes for preparing tissue products that have a majority of their surface containing a hydrophobic additive, yet have selective regions of hydrophobicity and hydrophilicity.